An organic light emitting device (OLED) consists of a cathode, an anode, a hole transport layer disposed between the cathode and the anode, a light emitting layer and an electron transport layer. When a proper voltage is applied to the anode and the cathode, the holes generated at the anode and the electrons generated at the cathode are transported through the hole transport layer and the electron transport layer respectively and recombined in the light emitting layer, so that the light emitting layer emits lights of three primary colors RGB which constitute basic colors of light emitting device based on compositions of the light emitting layer.
If electron mobility of the electron transport layer is low or the energy barrier for the electron to be injected into the electron transport layer from the cathode is high, the number of the electrons in the light emitting layer of the OLED decreases, thus the number of holes is greater than the number of electrons, and the number of holes and the number of electrons do not match, which results in a low light emitting efficiency of the OLED.